1. Field of the Invention
The present invention relates, in general, to a method for detecting microscopic differences in the thickness of a photoresist film coated on a wafer. More particularly, the present invention is concerned with a method for detecting the microscopic differences visibly such that the topology of the photoresist film can be improved easily.
2. Description of the Prior Art
As the design rule is smaller or the relative size of a wafer is smaller, which is attributed to the high integration of devices, it is more difficult to maintain uniformly the critical dimension (hereinafter referred to as "CD") of the patterns which are formed on many dies all over the wafer by lithography. In addition, microscopic differences in the thickness of the photoresist film coated on the wafer causes a difference in standing wave effect in spite of the same exposure energy, resulting in an alteration of the CD of each of the dies formed over the wafer. The amount of standing wave effect (S) is represented as Equation I: EQU S=C(R.sub.1 .times.R.sub.2).sup.1/2 e.sup.-.alpha.D I!
wherein C stands for constant;
R.sub.1 and R.sub.2 each are the intensity of a light scattered from the boundary between the bottom of a photoresist film and the upper surface of the wafer;
a is a light transmittivity parameter associated with the components of the photoresist film; and
D is the thickness of the photoresist film.
Because the amount of standing wave effect (S) has a proportional relation as represented in Equation I, the change of S value arises dependently on the R.sub.1 and R.sub.2 values with the same period with the thickness in the photoresist film.
With reference to FIG. 1, there is shown a curve of CD with respect to thickness of a photoresist film coated on a wafer. As shown in this curve, CD values are periodically changed. The change of CD value is dependent on the values of R.sub.1 and R.sub.2. In the figure, reference character "t" is a coating thickness difference of photoresist film which corresponds to a period of the standing wave effect. The coating thickness difference of photoresist film is proportional to .lambda./4n wherein .lambda. is a wavelength and n is a refractive index. Thus, the coating thickness difference of photoresist film (t) is microscopically change din a highly integrated device which has been exposed to a light source with a short wavelength, thereby changing the CD value. For 256 M DRAM (mega dynamic random access memory) an excimer laser having a wavelength of 248 nm is used for a photoresist film with a refractive index ranging from 1.6 to 1.7.
Generally speaking, spin-coat processes show their own characteristic differences in the coating thickness of photoresist film. The differences according to the spin-coat processes are unable to be measured by naked eyes. This is possible with a special measuring apparatus. However, there arise problems when measuring the coating state by means of such apparatus. Because measurements at many locations of the coating, which may be necessary for the accurate examination into the coating state of photoresist film, cannot be carried out or recognition of the outline of the coating shape is not possible, it is difficult to appropriately control the rotational speed, the amount of rinse for the photoresist film, the acceleration and the annealing step prior to the exposure when carrying out the coating process, resulting in obstructing the high integration of devices.